LRO Albedo and Thermal IR (Simple)

This example demonstrates how to compute albedo and thermal infrared accelerations using the object-oriented interface of the pyRTX library.

In this simplified case, we use single uniform values for:

• Planet emissivity

• Planet albedo

• Planet surface temperature

This provides a quick way to compute planetary radiation pressure effects without requiring detailed surface property maps.

lro_alb_ir_simple.py

### ------------------------------------------------------------------------------------------------------- ###

# Example purpose:
# Show the object-oriented interface of the pyRTX library
#
# Example case:
# Show how to compute albedo and thermal-ir accelerations
# Here we use a single value for the emissivity, albedo and temperature for the planet.

### ------------------------------------------------------------------------------------------------------- ###
### IMPORTS

import numpy as np
import spiceypy as sp
import matplotlib.pyplot as plt
import timeit

from pyRTX.classes.Spacecraft import Spacecraft
from pyRTX.classes.Planet import Planet
from pyRTX.classes.Radiation import Albedo, Emissivity
from pyRTX.classes.Precompute import Precompute
from pyRTX.classes.LookUpTable import LookUpTable
from pyRTX.core.analysis_utils import epochRange2

from numpy import floor, mod

### ------------------------------------------------------------------------------------------------------- ###
### INPUTS

# NOTE: before running this script you should generate the input lutfile running the 
# example 'compute_lut.py' using type = 'accel'.

ref_epc		=  "2010 may 10 09:25:00"
duration    =  10000  									  # seconds
timestep    =  100
METAKR      = '../example_data/LRO/metakernel_lro.tm'     # metakernel
obj_path    = '../example_data/LRO/'				      # folder with .obj files
lutfile     = 'luts/lro_accel_lut.nc'					  # lookup table file
base_flux   =  1361.5
ref_radius  =  1737.4
n_cores     =  10
sc_mass		=  2000  									  # can be a float, int or xarray [kg]

### ------------------------------------------------------------------------------------------------------- ###
### OBJECTS DEFINITION

# Time initialization
tic = timeit.default_timer()

# Load the metakernel containing references to the necessary SPICE frames
sp.furnsh(METAKR)

# Define epochs
epc_et0  = sp.str2et( ref_epc )
epc_et1  = epc_et0 + duration
epochs   = epochRange2(startEpoch = epc_et0, endEpoch = epc_et1, step = timestep)
 
# Define the Spacecraft Object (Refer to the class documentation for further details)
lro = Spacecraft( name = 'LRO',
                 
				  base_frame = 'LRO_SC_BUS', 					     # Name of the spacecraft body-fixed frame
      
                  mass = sc_mass,
      
				  spacecraft_model = {						         # Define a spacecraft model
                          
					'LRO_BUS': { 
							 'file' : obj_path + 'bus_rotated.obj',	 # .obj file of the spacecraft component
							 'frame_type': 'Spice',				     # type of frame (can be 'Spice' or 'UD'
							 'frame_name': 'LRO_SC_BUS',			 # Name of the frame
							 'center': [0.0,0.0,0.0],			     # Origin of the component
							 'diffuse': 0.1,				         # Diffuse reflect. coefficient
							 'specular': 0.3,				         # Specular reflect. coefficient
							 },

					'LRO_SA': {	
							'file': obj_path + 'SA_recentred.obj',
							'frame_type': 'Spice',
							'frame_name': 'LRO_SA',
							'center': [-1,-1.1, -0.1],
							'diffuse': 0,
							'specular': 0.3,
							},


					'LRO_HGA': { 	
							'file': obj_path + 'HGA_recentred.obj',
							'frame_type': 'Spice',
							'frame_name': 'LRO_HGA',
							'center':[-0.99,    -0.3,  -3.1],
							'diffuse': 0.2,
							'specular': 0.1,
							},
					}
					)

# Define the Moon object
moon = Planet(  fromFile = None,
                radius = ref_radius,
                name = 'Moon',
                bodyFrame = 'MOON_PA',
                sunFixedFrame = 'GSE_MOON',
                units = 'km',
                subdivs = 5,
                )

# Set the albedo and emissivity values
# Here we use dummy values and assume that
# albedo and emissivity are constant over the whole planet
# and set a different dayside and nightside temperature
# pyRTX supports also gridded values for albedo and emissivity (see examples/lro_alb_ir_grid.py)

moon.albedo = 0.3
moon.emissivity  = 0.8 
moon.dayside_temperature = 300
moon.nightside_temperature = 200

# Load the Look up table
LUT  = LookUpTable(lutfile)

# Precomputation object
prec = Precompute(epochs = epochs,)
prec.precomputePlanetaryRadiation(lro, moon, LUT.moving_frames, correction='CN')
prec.dump()

# Create the albedo object
albedo = Albedo(lro, LUT, moon, precomputation  = prec, baseflux  = base_flux,)

# Create the thermal infrared object
thermal_ir = Emissivity(lro, LUT, moon, precomputation  = prec, baseflux  = base_flux,)

### ------------------------------------------------------------------------------------------------------- ###
### COMPUTATIONS

# Both the albedo and emissivity objects have a .compute() method.
# This method returns the normalized fluxes, direction and albedo values
# for each of the planet faces contributing to the computation
# The general formula for computing the acceleration of an elementary face is:
#
# acc_i = L * albedo_value/mass * norm_flux
#
# where L is the normalized optical response of the spacecraft which can be computed
# with raytracing setting a unitary radiance of the impacting rays
# Due to the very high number of rays involved in these computations
# it is mandatory to precompute L in the form of a lookup table.
# Here we import the lookup table for LRO which can be computed using the example script
# 'compute_lut.py'

# Parallel computations
alb_accel = albedo.compute(epochs, n_cores = n_cores)[0] * 1e3 
ir_accel  = thermal_ir.compute(epochs, n_cores = n_cores)[0] * 1e3 

# Always unload the SPICE kernels
sp.unload(METAKR)

### ... Elapsed time
toc = timeit.default_timer()
time_min = int(floor((toc-tic)/60))
time_sec = int(mod((toc-tic), 60))
print("")
print("\t Elapsed time: %d min, %d sec" %(time_min, time_sec))
print("")

### ------------------------------------------------------------------------------------------------------- ###
### PLOT

epochs  = [float( epc - epc_et0 )/3600 for epc in epochs]

# ALBEDO 
fig, ax = plt.subplots(3, 1, figsize=(14,8), sharex = True)

ax[0].plot(epochs, alb_accel[:,0], linewidth = 2, color = "tab:blue")
ax[0].set_ylabel('X [m/s^2]')
ax[1].plot(epochs, alb_accel[:,1], linewidth = 2, color = "tab:blue")
ax[1].set_ylabel('Y [m/s^2]')
ax[2].plot(epochs, alb_accel[:,2], linewidth = 2, color = "tab:blue")
ax[2].set_ylabel('Z [m/s^2]')
ax[2].set_xlabel('Hours from CA')
fig.suptitle('Albedo in S/C body frame')
plt.tight_layout()

# IR
fig, ax = plt.subplots(3, 1, figsize=(14,8), sharex = True)

ax[0].plot(epochs, ir_accel[:,0], linewidth = 2, color = "tab:blue")
ax[0].set_ylabel('X [m/s^2]')
ax[1].plot(epochs, ir_accel[:,1], linewidth = 2, color = "tab:blue")
ax[1].set_ylabel('Y [m/s^2]')
ax[2].plot(epochs, ir_accel[:,2], linewidth = 2, color = "tab:blue")
ax[2].set_ylabel('Z [m/s^2]')
ax[2].set_xlabel('Hours from CA')
fig.suptitle('IR in S/C body frame')
plt.tight_layout()

plt.show()

### ------------------------------------------------------------------------------------------------------- ###